Carbon nanotube fiber and method for producing same

ABSTRACT

A carbon nanotube fiber is provided that that has excellent properties such as electrical conductivity, thermal conductivity, and mechanical characteristics. The carbon nanotube fiber includes an assembly of a plurality of carbon nanotubes. The plurality of carbon nanotubes includes one or more carbon nanotubes having at least partially collapsed structures. Furthermore, a method for producing a carbon nanotube fiber is provided that includes spinning a carbon nanotube dispersion liquid containing a plurality of carbon nanotubes including one or more carbon nanotubes having at least partially collapsed structures, a dispersant, and a solvent by extruding the carbon nanotube dispersion liquid into a coagulant liquid.

TECHNICAL FIELD

The present disclosure relates to a carbon nanotube fiber and a methodfor producing the same, and in particular relates to a carbon nanotubefiber including an assembly of a plurality of carbon nanotubes and amethod for producing this carbon nanotube fiber.

BACKGROUND

In recent years, carbon nanotubes (hereinafter, also referred to as“CNTs”) have been attracting attention as materials having excellentelectrical conductivity, thermal conductivity, and mechanicalcharacteristics.

However, CNTs are fine tubular structures having nanometer-sizediameters, which makes handling and processing of individual CNTsdifficult. In consideration of this, it has for example been proposedthat a plurality of CNTs may be assembled into a fibrous shape to form acarbon nanotube fiber (hereinafter, also referred to as a “CNT fiber”),and this CNT fiber may be used individually or may be used as part of ayarn (for example, refer to PTL 1 and 2). Furthermore, there is muchinterest in using such CNT fibers as fibrous materials having excellentelectrical conductivity, thermal conductivity, and mechanicalcharacteristics.

CITATION LIST Patent Literature

-   PTL 1: JP 2011-038203 A-   PTL 2: JP 2012-127043 A

SUMMARY Technical Problem

There is demand for further improvement in properties (electricalconductivity, thermal conductivity, mechanical characteristics, and soforth) of CNT fibers. In order to improve such properties of CNT fibers,it is important to cause high-density assembly of CNTs that haveexcellent characteristics. However, in the case of conventional CNTfibers, it has not been possible to cause high-density assembly of CNTshaving excellent characteristics. Therefore, these conventional CNTfibers leave room for further improvement in terms of properties.

An objective of the present disclosure is to provide a carbon nanotubefiber that has excellent properties such as electrical conductivity,thermal conductivity, and mechanical characteristics.

Solution to Problem

The inventor conducted diligent investigation in order to achieve theobjective described above. Through this investigation, the inventordiscovered that a carbon nanotube fiber having excellent properties suchas electrical conductivity, thermal conductivity, and mechanicalcharacteristics can be obtained by forming the carbon nanotube fiberusing carbon nanotubes among which at least some have a specificstructure. This discovery led to the present disclosure.

Specifically, in order to beneficially solve the problem described abovethrough the present disclosure, a presently disclosed carbon nanotubefiber includes an assembly of a plurality of carbon nanotubes, whereinthe plurality of carbon nanotubes includes one or more carbon nanotubeshaving at least partially collapsed structures. As a result of thecarbon nanotube fiber being formed using one or more carbon nanotubeshaving at least partially collapsed structures, the carbon nanotubefiber can be provided with excellent properties such as electricalconductivity, thermal conductivity, and mechanical characteristics.

In the presently disclosed carbon nanotube fiber, the one or more carbonnanotubes having at least partially collapsed structures are preferablypresent in a proportion of at least 5 in 100 carbon nanotubes. Thereason for this is that when the one or more carbon nanotubes having atleast partially collapsed structures are present in a proportion of atleast 5 in 100 carbon nanotubes, properties such as electricalconductivity, thermal conductivity, and mechanical characteristics canbe sufficiently improved.

In the presently disclosed carbon nanotube fiber, the one or more carbonnanotubes having at least partially collapsed structures preferably havesingle-walled structures. The reason for this is that when the one ormore carbon nanotubes having at least partially collapsed structureshave single-walled structures, properties of the carbon nanotube fibersuch as electrical conductivity, thermal conductivity, and mechanicalcharacteristics can be further improved.

In the presently disclosed carbon nanotube fiber, the one or more carbonnanotubes having at least partially collapsed structures preferably eachhave a section into which fullerenes are not inserted upon beingsubjected to fullerene insertion treatment. The reason for this is thatwhen the one or more carbon nanotubes having at least partiallycollapsed structures each have a section into which fullerenes are notinserted, properties of the carbon nanotube fiber such as electricalconductivity, thermal conductivity, and mechanical characteristics canbe further improved.

In the presently disclosed carbon nanotube fiber, an average width ofcollapsed sections in the one or more carbon nanotubes having at leastpartially collapsed structures is preferably at least 5 nm and nogreater than 9 nm. The reason for this is that when the average width ofcollapsed sections in the one or more carbon nanotubes having at leastpartially collapsed structures is at least 5 nm and no greater than 9nm, properties of the carbon nanotube fiber such as electricalconductivity, thermal conductivity, and mechanical characteristics canbe further improved.

In the presently disclosed carbon nanotube fiber, the plurality ofcarbon nanotubes preferably has a BET specific surface area of at least600 m²/g. The reason for this is that when the plurality of carbonnanotubes, which includes the one or more carbon nanotubes having atleast partially collapsed structures, has a BET specific surface area ofat least 600 m²/g, properties of the carbon nanotube fiber such aselectrical conductivity, thermal conductivity, and mechanicalcharacteristics can be further improved.

The presently disclosed carbon nanotube fiber preferably has a densityof at least 1.0 g/cm³ and no greater than 1.5 g/cm³. The reason for thisis that when the density is at least 1.0 g/cm³ and no greater than 1.5g/cm³, properties such as electrical conductivity, thermal conductivity,and mechanical characteristics can be sufficiently improved and thecarbon nanotube fiber can be easily produced.

Furthermore, in order to beneficially solve the problem described abovethrough the present disclosure, a presently disclosed method forproducing a carbon nanotube fiber includes spinning a carbon nanotubedispersion liquid containing a plurality of carbon nanotubes includingone or more carbon nanotubes having at least partially collapsedstructures, a dispersant, and a solvent by extruding the carbon nanotubedispersion into a coagulant liquid. The carbon nanotube fiber producedby this method includes the one or more carbon nanotubes having at leastpartially collapsed structures and has excellent properties such aselectrical conductivity, thermal conductivity, and mechanicalcharacteristics.

The presently disclosed method for producing a carbon nanotube fiberpreferably further includes preparing the carbon nanotube dispersionliquid by subjecting a coarse dispersion liquid containing the pluralityof carbon nanotubes, the dispersant, and the solvent to dispersiontreatment that brings about a cavitation effect or a crushing effect inorder to disperse the plurality of carbon nanotubes. The reason for thisis that when the carbon nanotube dispersion liquid is prepared bydispersion treatment that brings about a cavitation effect or a crushingeffect, properties of the produced carbon nanotube fiber such aselectrical conductivity, thermal conductivity, and mechanicalcharacteristics can be sufficiently improved.

The presently disclosed carbon nanotube fiber described above can befavorably produced by the presently disclosed method for producing acarbon nanotube fiber described above.

Advantageous Effect

According to the present disclosure, a carbon nanotube fiber can beprovided that has excellent properties such as electrical conductivity,thermal conductivity, and mechanical characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a TEM image of CNTs after being subjected to fullereneinsertion treatment; and

FIG. 2 is an enlarged image showing an enlargement of part of the TEMimage in FIG. 1.

DETAILED DESCRIPTION

The following provides a detailed description of embodiments of thepresent disclosure.

A presently disclosed carbon nanotube fiber includes one or more carbonnanotubes having at least partially collapsed structures. The presentlydisclosed carbon nanotube fiber can be produced by a presently disclosedmethod for producing a carbon nanotube fiber.

The presently disclosed carbon nanotube fiber can be used individuallyor as part of a carbon nanotube yarn in which a plurality of carbonnanotube fibers are intertwined.

(Carbon Nanotube Fiber)

The presently disclosed carbon nanotube fiber is composed of a carbonnanotube (CNT) assembly formed by causing a plurality of CNTs toassemble in a fibrous shape. One main feature of the presently disclosedCNT fiber is that all or some of the plurality of CNTs composing theassembly are carbon nanotubes having at least partially collapsedstructures (hereinafter, also referred to as “collapsed CNTs”). Thepresently disclosed CNT fiber has excellent properties such aselectrical conductivity, thermal conductivity, and mechanicalcharacteristics as a result of including the collapsed CNT.

In the present disclosure, the carbon nanotube fiber may be amonofilament composed of a single filament (CNT filament) or amultifilament composed of a plurality of filaments.

Although the reason that properties of the CNT fiber are improved as aresult of the collapsed CNT being included among the plurality of CNTsthat are used is not clear, it is presumed that properties of the CNTfiber are improved due to the CNTs being able to assemble more denselythan when only CNTs that do not have a collapsed structure are used andalso due to the collapsed CNTs themselves having excellentcharacteristics.

<Carbon Nanotubes>

The plurality of CNTs that compose the CNT fiber are required to includeone or more collapsed CNTs. It is presumed that when collapsed CNTs areused in a CNT fiber or the like, excellent characteristics can bedisplayed and high-density assembly can be achieved as a result of thecollapsed CNTs having a different structure to normal circulartube-shaped CNTs in terms of external shape and internal space.

[Carbon Nanotube Having at Least Partially Collapsed Structures]

Herein, when CNTs are described as “having at least partially collapsedstructures” this refers to the fact that when such CNTs are sealed in aquartz tube with fullerenes (C₆₀) and are subjected to heat treatment atreduced pressure (fullerene insertion treatment), the resultantfullerene inserted-CNTs, when observed using a transmission electronmicroscope (TEM), each have a section in which fullerenes are notinserted.

For example, in proximity to a location indicated by an arrow in a TEMimage in FIG. 1, which is enlarged in FIG. 2, fullerenes are onlyinserted at opposite edges of the shown CNT in a width direction (i.e.,direction perpendicular to an extension direction of the CNT) and arenot inserted other than at the edges. Accordingly, it can be determinedthat the CNT has a collapsed section in which fullerenes are notinserted, and therefore has a collapsed structure.

It should be noted that with regards to the “CNTs having at leastpartially collapsed structures”, one “collapsed structure” or aplurality of “collapsed structures” may be present in a single CNT.

Although the structure of collapsed CNTs is not clear, a structure isfor example envisaged that is a tube formed by rolled up graphene andthat has at least a section where cross-sectional shape perpendicular toan extension direction (axial direction) of the tube is non-circular.The cross-sectional shape of the collapsed CNT is more preferably ashape in which the maximum width in a direction perpendicular to alongitudinal direction of the cross-section is greater in proximity toboth longitudinal direction edges of the cross-section than in proximityto a longitudinal direction central part of the cross-section, and isparticularly preferably a dumbbell shape (i.e., a shape in which thelongitudinal direction central part of the cross-section is collapsed).

With regards to cross-sectional shape of the collapsed CNT, “inproximity to a longitudinal direction central part of the cross-section”refers to a region within 30% of cross-section longitudinal directionlength of a longitudinally central line in the cross-section (i.e., astraight line running centrally through the cross-section in terms ofthe longitudinal direction and perpendicularly intersecting alongitudinal direction line), and “in proximity to a longitudinaldirection edge of the cross-section” refers to a region further outwardin the longitudinal direction than “in proximity to a longitudinaldirection central part of the cross-section”.

The one or more collapsed CNTs preferably have at least partiallycollapsed structures from the point at which the CNTs are synthesized.The inventor presumes, based on research, that a carbon nanotube havingan at least partially collapsed structure from the point of synthesis(collapsed CNT) has significantly different properties to a normalcarbon nanotube having a circular tube-shaped structure or a carbonnanotube that does not have a collapsed structure from the point ofsynthesis but that has undergone structural deformation after beingformed with a circular tube-shaped structure. Specifically, it ispresumed that a collapsed CNT that has an at least partially collapsedstructure from the point of synthesis is a material formed by a networkof six member rings in which carbon atoms are sp² bonded to one anothersuch as to have a “collapsed structure” as described above, and thismaterial is considered to be a novel material different from anycommonly known structures composed of carbon.

Herein, an average width of collapsed sections in the one or morecollapsed CNTs (i.e., sections in which fullerenes are not inserted infullerene insertion treatment) is preferably at least 5 nm and nogreater than 9 nm. The reason for this is that when the average width ofthe collapsed sections in the CNTs is at least 5 nm and no greater than9 nm, properties of the CNT fiber can be further improved.

In the present disclosure, the “average width of collapsed sections inthe CNTs” is an arithmetic mean value obtained by measuring the lengthsof collapsed sections in a CNT width direction with respect to 10 randomCNTs having collapsed structures using a transmission electronmicroscope. The lengths in the width direction of the collapsed CNTs arepreferably distributed in a range from 1 nm to 10 nm.

[Plurality of Carbon Nanotubes]

The plurality of CNTs that include the previously described one or morecollapsed CNTs and that compose the CNT fiber may, without any specificlimitations, have a single-walled structure or a multi-walled structure,but preferably have a single-walled structure. In other words, theplurality of CNTs including the one or more collapsed CNTs arepreferably single-walled carbon nanotubes. In particular, properties ofthe CNT fiber can be favorably improved in a situation in which the oneor more collapsed CNTs have a single-walled structure.

In a Raman spectrum of the plurality of CNTs, a ratio of G band peakintensity relative to D band peak intensity (G/D ratio) is preferably atleast 1, and is preferably no greater than 50, and more preferably nogreater than 10. A G/D ratio of no greater than 10 indicates that alarge number of amorphous locations are present. The G/D ratio is anindex commonly used to evaluate the quality of CNTs. In a Raman spectrumof CNTs measured by a Raman spectrometer, vibration modes referred to asa G band (near 1600 cm⁻¹) and a D band (near 1350 cm⁻¹) are observed.The G band is a vibration mode based on hexagonal lattice structure ofgraphite and the D band is a vibration mode based on amorphouslocations. CNTs having a high ratio of G band and D band peakintensities (G/D ratio) can be evaluated as having high crystallinity.

A BET specific surface area of the plurality of CNTs is preferably atleast 600 m²/g, and more preferably at least 800 m²/g, and is preferablyno greater than 1,400 m²/g, and more preferably no greater than 1,200m²/g. The reason for this is that when the BET specific surface area ofthe plurality of CNTs is at least 600 m²/g, properties of the CNT fibercan be sufficiently improved. Furthermore, when the BET specific surfacearea of the plurality of CNTs is no greater than 1,400 m²/g, thenegative effect of CNT agglomeration on properties of the CNT fiber canbe suppressed.

Note that the “BET specific surface area” can be obtained by the BETmethod through measurement of a nitrogen adsorption isotherm at 77 K.Herein, the BET specific surface area can be measured using, forexample, a BELSORP®-max (BELSORP is a registered trademark in Japan,other countries, or both) produced by Bel Japan Inc.

The length of the plurality of CNTs at the time of production ispreferably at least 100 μm and no greater than 5,000 μm.

The plurality of CNTs preferably includes the one or more collapsed CNTsin a proportion of at least 5 collapsed CNTs per 100 CNTs, morepreferably in a proportion of at least 10 collapsed CNTs per 100 CNTs,further preferably in a proportion of at least 20 collapsed CNTs per 100CNTs, and particularly preferably in a proportion of at least 30collapsed CNTs per 100 CNTs. The reason for this is that when thecollapsed CNTs are contained in a proportion of at least 5 in 100 CNTs,properties of the CNT fiber can be sufficiently improved.

In the present disclosure, the “proportion of collapsed CNTs” can beobtained by observing 100 random CNTs using a transmission electronmicroscope and counting the number of collapsed CNTs present among the100 CNTs.

[Production Method of Plurality of Carbon Nanotubes]

The plurality of CNTs including the one or more collapsed CNTs can beproduced by synthesizing a plurality of CNTs that includes collapsedCNTs or can be produced by separately synthesizing collapsed CNTs andgeneric CNTs (circular tube-shaped CNTs), and subsequently mixing thecollapsed CNTs and the generic CNTs.

The following describes, as one example, a production method in which aplurality of CNTs that includes one or more collapsed CNTs issynthesized.

The production method in which the plurality of CNTs that includes oneor more collapsed CNTs is synthesized makes use of a CVD method andspecifically includes at least:

(1) a step of applying a coating liquid A containing an aluminumcompound onto a substrate;

(2) a step of drying the coating liquid A to form an aluminum thin filmon the substrate;

(3) a step of applying a coating liquid B containing an iron compoundonto the aluminum thin film;

(4) a step of drying the coating liquid B at a temperature of no higherthan 50° C. to form an iron thin film on the aluminum thin film andthereby obtain a catalyst substrate; and

(5) a step of growing carbon nanotubes on the catalyst substrate bysuppling a feedstock gas to the catalyst substrate (growth step).

Hereinafter, steps (1) and (2) are collectively referred to as a“catalyst supporting layer formation step” and steps (3) and (4) arecollectively referred to as a “catalyst layer formation step”.

According to the production method described above, as a result of thecatalyst substrate being prepared by a wet process and the dryingtemperature when obtaining the catalyst layer (iron thin film) by dryingbeing no higher than 50° C., CNTs that include one or more collapsedCNTs can be produced from initial production.

[[Catalyst Supporting Layer Formation Step]]

First, the coating liquid A containing the aluminum compound is appliedonto the substrate and is dried to form the aluminum thin film on thesubstrate. The aluminum thin film formed on the substrate as describedabove functions as a catalyst supporting layer on which the iron thinfilm (catalyst layer) explained below is supported.

—Substrate—

The substrate used for the catalyst substrate is for example a flatplate-shaped member and can preferably maintain shape up to a hightemperature of at least 500° C. Specific examples of the substrateinclude metals such as iron, nickel, chromium, molybdenum, tungsten,titanium, aluminum, manganese, cobalt, copper, silver, gold, platinum,niobium, tantalum, lead, zinc, gallium, indium, germanium, and antimony,alloys and oxides including these metals, non-metals such as silicon,quartz, glass, mica, graphite, and diamond, and ceramics. Metalmaterials are preferable due to their low cost and ease of processingcompared to silicon and ceramics, and particularly suitable examplesinclude Fe—Cr (iron-chromium) alloy, Fe—Ni (iron-nickel) alloy, andFe—Cr—Ni (iron-chromium-nickel) alloy.

No specific limitations are placed on the thickness of the substrate,which can for example range from a thin film having a thickness ofseveral micrometers to a plate having a thick of several centimeters.The thickness of the substrate is preferably at least 0.05 mm and nogreater than 3 mm.

Although no specific limitations are placed on the area of thesubstrate, the area is preferably at least 20 cm², and more preferablyat least 30 cm². The shape of the substrate can for example berectangular or square, but is not specifically limited.

—Coating Liquid A—

The coating liquid A is a liquid in which the aluminum compound isdissolved or dispersed in an organic solvent. No specific limitationsare placed on the aluminum compound contained in the coating liquid Aother than being a compound that includes an aluminum atom. The aluminumcompound is preferably an organometallic compound or a metal salt thatcan be used to form an alumina thin film as the aluminum thin film.

Examples of organometallic compounds that can be used to form an aluminathin film include aluminum alkoxides such as aluminum trimethoxide,aluminum triethoxide, aluminum tri-n-propoxide, aluminumtri-i-propoxide, aluminum tri-n-butoxide, aluminum tri-sec-butoxide, andaluminum tri-tert-butoxide. Other examples of aluminum-containingorganometallic compounds include complexes such astris(acetylacetonato)aluminum(III). Examples of metal salts that can beused to form an alumina thin film include aluminum sulfate, aluminumchloride, aluminum nitrate, aluminum bromide, aluminum iodide, aluminumlactate, basic aluminum chloride, and basic aluminum nitrate. Any ofsuch examples can be used individually or as a mixture.

Various organic solvents such as alcohols, glycols, ketones, ethers,esters, and hydrocarbons can be used as the organic solvent contained inthe coating liquid A. However, an alcohol or a glycol is preferably usedas the solvent because organometallic compounds and metal salts havegood solubility therein. These organic solvents may be used individuallyor as a mixture of two or more types of organic solvents. Examples ofalcohols that are preferable in terms of handleability and preservationstability include methanol, ethanol, and isopropyl alcohol.

A stabilizer may be added to the coating liquid A in order to suppress acondensation polymerization reaction of the organometallic compound andthe metal salt. The stabilizer is preferably at least one selected fromthe group consisting of β-diketones and alkanolamines. Examples ofβ-diketones that can be used include acetylacetone, methyl acetoacetate,ethyl acetoacetate, benzoylacetone, dibenzoylmethane,benzoyltrifluoroacetone, furoylacetone, and trifluoroacetylacetone, withacetylacetone and ethyl acetoacetate being particularly preferable.Examples of alkanolamines that can be used include monoethanolamine,diethanolamine, triethanolamine, N-methyldiethanolamine,N-ethyldiethanolamine, N,N-dimethylaminoethanol, diisopropanolamine, andtriisopropanolamine, with a secondary or tertiary alkanolamine beingpreferable.

Although no specific limitations are placed on the amount of thealuminum compound in the coating liquid A, the amount per 100 mL of theorganic solvent is preferably at least 0.1 g, and more preferably atleast 0.5 g, and is preferably no greater than 30 g, and more preferablyno greater than 5 g.

Furthermore, although no specific limitations are placed on the amountof the stabilizer in the coating liquid A, the amount per 100 mL of theorganic solvent is preferably at least 0.01 g, and more preferably atleast 0.1 g, and is preferably no greater than 20 g, and more preferablyno greater than 3 g.

—Application—

The coating liquid A described above is applied onto the substrate. Nospecific limitations are placed on the method by which the coatingliquid A is applied onto the substrate. Although the coating liquid Acan for example be applied by a method involving application by sprayingor brushing, or by spin coating, dip coating, or the like, dip coatingis preferable from a viewpoint of productivity and film thicknesscontrol.

Dip coating is a method in which an application target (in this case,the coating liquid A) is applied onto the surface of a substrate byimmersing the substrate in the application target for a fixed time andsubsequently pulling up the substrate.

—Drying—

Next, the coating liquid A is dried on the substrate to form thealuminum thin film (catalyst supporting layer) on the substrate.Although no specific limitations are placed on the method by which thecoating liquid A is dried on the substrate, the coating liquid A can forexample be dried by air drying at room temperature or by heating(sintering process), with heating being preferable. The heatingtemperature is preferably approximately 50° C. to 400° C., and is morepreferably no higher than 350° C. The heating time is preferably atleast 5 minutes and no greater than 60 minutes, and is more preferablyno greater than 40 minutes.

[[Catalyst Layer Formation Step]]

Next, the coating liquid B containing the iron compound is applied ontothe aluminum thin film formed in the catalyst supporting layer formationstep, and is dried at a temperature of no higher than 50° C. to form theiron thin film on the aluminum thin film. As a result of this step, thecatalyst substrate having the aluminum thin film (catalyst supportinglayer) and the iron thin film (catalyst layer) on the substrate can beobtained.

—Coating Liquid B—

The coating liquid B is a liquid in which the iron compound is dissolvedor dispersed in an organic solvent. No specific limitations are placedon the iron compound contained in the coating liquid B other than beinga compound that includes an iron atom. The iron compound is preferablyan organometallic compound or a metal salt that can be used to form aniron thin film.

Examples of organometallic compounds that can be used to form an ironthin film include iron pentacarbonyl, ferrocene, iron(II)acetylacetonate, iron(III) acetylacetonate, iron(II)trifluoroacetylacetonate, and iron(III) trifluoroacetylacetonate.Examples of metal salts that can be used to form an iron thin filminclude inorganic acid iron salts such as iron sulfate, iron nitrate,iron phosphate, iron chloride, and iron bromide, and organic acid ironsalts such as iron acetate, iron oxalate, iron citrate, and ironlactate. From among these examples, use of an organic acid iron salt ispreferable. These examples can be used individually or as a mixture.

No specific limitations are placed on the organic solvent contained inthe coating liquid B. The organic solvent can be any of the organicsolvents described in the previous section relating to the coatingliquid A. Moreover, the coating liquid B may contain any of thestabilizers described in the previous section relating to the coatingliquid A.

Although no specific limitations are placed on the amount of the ironcompound in the coating liquid B, the amount per 100 mL of the organicsolvent is preferably at least 0.05 g, and more preferably at least 0.1g, and is preferably no greater than 5 g, and more preferably no greaterthan 1 g.

Furthermore, although no specific limitations are placed on the amountof the stabilizer in the coating liquid B, the amount per 100 mL of theorganic solvent is preferably at least 0.05 g, and more preferably atleast 0.1 g, and is preferably no greater than 5 g, and more preferablyno greater than 1 g.

—Application—

No specific limitations are placed on the method by which the coatingliquid B is applied onto the aluminum thin film. The coating liquid Bcan be applied by any of the methods described in the previous sectionrelating to the catalyst supporting layer formation step.

In the same way as for application of the coating liquid A in thepreviously described catalyst supporting layer formation step, dipcoating is preferably used as the application method of the coatingliquid B.

In a situation in which dip coating is used, the immersion time of thealuminum thin film-equipped substrate in the coating liquid B ispreferably at least 1 s and no greater than 30 s. Furthermore, thepulling up speed of the substrate from the coating liquid B afterimmersion is preferably at least 1 mm/s and no greater than 5 mm/s. Ifthe pulling up speed is greater than 5 mm/s, there may be insufficientadhesion of the coating liquid B to the substrate and the proportion ofcollapsed CNTs in the obtained plurality of CNTs may be reduced.

—Drying—

Next, the coating liquid B is dried on the aluminum thin film to formthe iron thin film on the substrate. Herein, it is necessary to performdrying of the coating liquid B at no higher than 50° C., preferably nohigher than 40° C., and more preferably no higher than 30° C. If thedrying temperature is higher than 50° C., it is not possible tosynthesize CNTs that include one or more collapsed CNTs in the followinggrowth step. Although no specific limitations are placed on the lowerlimit for the drying temperature, the drying temperature is normally atleast 10° C. Furthermore, it is normally preferable that the method bywhich the coating liquid B is dried on the substrate is air drying.Drying may be performed by heating so long as the drying temperature isno higher than 50° C. However, air drying is more suitable from aviewpoint of efficiently producing collapsed CNTs.

[[Formation Step]]

In the production method of the CNTs that include one or more collapsedCNTs, a formation step may be carried out before the growth step. Theformation step is a step of providing a reducing gas (reductive gas)environment as an environment around the catalyst and heating either orboth of the catalyst and the reducing gas. The formation step bringsabout one or more effects among reduction of the catalyst, promotion ofmicronization of the catalyst to a state suitable for CNT growth, andimprovement of catalyst activity. For example, in a situation in whichthe catalyst substrate includes an alumina-iron thin film composed of analumina thin film and an iron thin film, the iron catalyst is reducedand micronized such that a large number of nanometer-size iron fineparticles are formed on the alumina thin film (catalyst supportinglayer). As a result, the iron thin film (catalyst layer) is placed in asuitable state for CNT production. Although CNTs can be produced even ifthe formation step is omitted, the amount and quality of CNTs that areproduced can be dramatically improved by carrying out the formationstep.

—Reducing Gas—

Examples of gases that can be used as the reducing gas in the formationstep include hydrogen gas, ammonia, water vapor, and mixed gasesthereof. Furthermore, the reducing gas may be a mixed gas obtained bymixing hydrogen gas with an inert gas such as helium gas, argon gas, ornitrogen gas. The reducing gas may also be used in the growth step asappropriate.

The temperature of the catalyst and/or the reducing gas in the formationstep is preferably at least 400° C. and no greater than 1100° C. Theduration of the formation step is preferably at least 3 minutes and nogreater than 20 minutes, and is more preferably at least 3 minutes andno greater than 10 minutes. Through the above, it is possible tosuppress reduction in thickness of the iron thin film (catalyst layer)due to sintering proceeding during the formation step.

[[Growth Step]]

Next, a feedstock gas is supplied to the catalyst substrate that hasbeen obtained through the catalyst supporting layer formation step andthe catalyst layer formation step, and carbon nanotubes (i.e., analigned CNT aggregate) are grown on the catalyst substrate.

Either or both of the catalyst layer and the feedstock gas are normallyheated in the growth step. However, from a viewpoint of growing CNTswith uniform density, it is preferable that at least the feedstock gasis heated. The heating temperature is preferably at least 400° C. and nogreater than 1100° C. The growth step is carried out by introducing thefeedstock gas, an inert gas, and optionally either or both of a reducinggas and a catalyst activating material into a CNT growth furnace housingthe catalyst substrate.

From a viewpoint of increasing CNT production efficiency, it ispreferable that the reducing gas and the feedstock gas are supplied tothe catalyst on the catalyst substrate by a gas shower.

—Feedstock Gas—

The feedstock gas is a carbon source-containing material that is a gasat the temperature of CNT growth. Among such gases, hydrocarbons such asmethane, ethane, ethylene, propane, butane, pentane, hexane, heptane,propylene, and acetylene are suitable. Other examples include low-carbonnumber oxygen-containing compounds such as acetone, carbon monoxide, andlower alcohols such as methanol and ethanol. Mixtures of any of theabove examples can also be used.

—Inert Gas—

The feedstock gas may be diluted with an inert gas. The inert gas is agas that is inert at the temperature of CNT growth and that does notreact with the grown CNTs, and is preferably a gas that does not reduceactivity of the catalyst. Examples of inert gases that can be usedinclude noble gases such as helium, argon, neon, and krypton; nitrogen;hydrogen; and mixed gases of any of these gases.

—Catalyst Activating Material—

A catalyst activating material may be added in the CNT growth step. CNTproduction efficiency and purity can be further improved throughaddition of the catalyst activating material. The catalyst activatingmaterial used herein is typically an oxygen-containing material and ispreferably a material that does not cause significant damage to CNTs atthe temperature of CNT growth. Examples of effective catalyst activatingmaterials include water, oxygen, ozone, acidic gases, nitrogen oxide,and low-carbon number oxygen-containing compounds such as carbonmonoxide and carbon dioxide; alcohols such as ethanol and methanol;ethers such as tetrahydrofuran; ketones such as acetone; aldehydes;esters; and mixtures of any of these materials. Among these examples,water, oxygen, carbon dioxide, carbon monoxide, and ethers arepreferable, and water is particularly suitable.

Although no specific limitations are placed on the volume concentrationof the catalyst activating material, a trace amount is preferable. Forexample, in a situation in which the catalyst activating material iswater, the volume concentration in a gas introduced into the furnace isnormally from 10 ppm to 10,000 ppm, and preferably from 50 ppm to 1,000ppm.

—Other Conditions—

Pressure in the reaction furnace and process time in the growth step areappropriately set in consideration of other conditions. For example, thepressure can be approximately 1×10² Pa to 1×10⁷ Pa and the process timecan be approximately 1 minute to 60 minutes.

[[Cooling Step]]

The production method for CNTs that include one or more collapsed CNTspreferably includes a cooling step after the growth step. The coolingstep is a step in which the aligned CNT aggregate and the catalystsubstrate are cooled in the presence of a cooling gas after the growthstep. After the growth step, the aligned CNT aggregate and the catalystsubstrate are at a high temperature, and therefore may oxidize uponbeing placed in the presence of oxygen. In order to prevent suchoxidation, the aligned CNT aggregate and the catalyst substrate arecooled in the presence of a cooling gas to, for example, 400° C. orlower, and more preferably to 200° C. or lower. The cooling gas ispreferably an inert gas and is particularly preferably nitrogen in termsof safety, cost, and so forth.

[[Peeling Step]]

The production method for CNTs that include one or more collapsed CNTspreferably includes a step in which the aligned CNT aggregate formed onthe catalyst substrate is peeled from the catalyst substrate (peelingstep). The aligned CNT aggregate can be peeled from the catalystsubstrate physically, chemically, or mechanically. Examples of methodsthat can be used include peeling by an electric field, a magnetic field,centrifugal force, or surface tension; direct mechanical peeling fromthe substrate; and peeling from the substrate by pressure or heating.One example of a simple peeling method involves peeling from thecatalyst substrate by direct pinching using tweezers. In a more suitableexample, the aligned CNT aggregate can be cut away from the catalystsubstrate using a thin blade such as a cutter blade. In another example,the aligned CNT aggregate can be sucked and peeled from the catalystsubstrate using a vacuum pump or cleaner. Note that the catalyst remainson the substrate after peeling of the CNTs and can be reused to onceagain to grow perpendicularly oriented CNTs.

[[Production Apparatus]]

No specific limitations are placed on the production apparatus used inthe CNT production method described above other than being an apparatusthat includes a growth furnace (reaction chamber) for housing thecatalyst substrate and that can be used to grow CNTs by CVD. Forexample, the production apparatus can be a thermal CVD furnace or anMOCVD reaction furnace.

The carbon purity of the CNTs obtained by the above-described productionmethod, without carrying out purification treatment, is preferably atleast 98 mass %, more preferably at least 99 mass %, and particularlypreferably at least 99.9 mass %. Purification treatment may be carriedout as desired. Note that the carbon purity can be obtained throughelemental analysis by X-ray fluorescence.

<Carbon Nanotube Fiber Properties>

Herein, the carbon nanotube fiber formed by the assembly of theplurality of CNTs described above preferably has the followingproperties.

[Carbon Nanotube Content]

Specifically, the presently disclosed CNT fiber is preferably composedof at least 75 mass % of CNTs and is more preferably composedsubstantially of only CNTs (i.e., does not include other componentsbesides incidental impurities that are mixed in during production). Thereasons for this is that when the CNT content is at least 75 mass %,characteristics of the CNTs can be favorably expressed, and propertiesof the CNT fiber such as electrical conductivity, thermal conductivity,and mechanical characteristics can be sufficiently improved.

[Density]

The density of the presently disclosed CNT fiber is preferably at least1.0 g/cm³, and more preferably at least 1.2 g/cm³, and is preferably nogreater than 1.5 g/cm³. The reason for this is that when the density ofthe CNT fiber is at least 1.0 g/cm³, properties such as electricalconductivity, thermal conductivity, and mechanical characteristics canbe sufficiently improved. Furthermore, when the density of the CNT fiberis no greater than 1.5 g/cm³, the CNT fiber can be easily produced.

In the present disclosure, the density of a carbon nanotube fiber can beobtained by measuring the mass, diameter, and length of a CNT filamentincluded in the CNT fiber, obtaining the volume of the CNT filament byassuming the CNT filament to have a cylindrical shape, and dividing themass of the CNT filament by the volume. In other words, in the presentdisclosure, when the CNT fiber is a multifilament including a pluralityof filaments (CNT filaments), the “density of the CNT fiber” refers tothe density of the CNT filaments included in the multifilament.

(Carbon Nanotube Fiber Production Method)

A presently disclosed method for producing a carbon nanotube fiber canbe used to produce the presently disclosed carbon nanotube fiberdescribed above. One main feature of the presently disclosed method forproducing a CNT fiber is that the method includes a step (spinning step)of spinning a carbon nanotube dispersion liquid (hereinafter, alsoreferred to as a “CNT dispersion liquid”) containing a plurality of CNTsincluding one or more CNTs having at least partially collapsedstructures, a dispersant, and a solvent by extruding the carbon nanotubedispersion into a coagulant liquid. The presently disclosed method forproducing a CNT fiber may include, prior to the spinning step, a step(dispersion liquid preparation step) of preparing the CNT dispersionliquid by subjecting a coarse dispersion liquid containing the pluralityof CNTs, the dispersant, and the solvent to dispersion treatment.

A CNT fiber obtained through the presently disclosed method forproducing a CNT fiber has excellent properties such as electricalconductivity, thermal conductivity, and mechanical characteristics as aresult of the CNT fiber containing the one or more collapsed CNTs.

<Dispersion Liquid Preparation Step>

In the dispersion liquid preparation step, the CNT dispersion liquid ispreferably prepared by subjecting a coarse dispersion liquid containingthe plurality of carbon nanotubes, the dispersant, and the solvent todispersion treatment that brings about a cavitation effect or a crushingeffect in order to disperse the carbon nanotubes. The reason for this isthat a CNT dispersion liquid in which CNTs are favorably dispersed canbe obtained by using dispersion treatment that brings about a cavitationeffect or a crushing effect. Furthermore, when a CNT fiber is preparedusing the CNT dispersion liquid in which the CNTs are favorablydispersed, CNTs having excellent characteristics can be caused touniformly assemble with high density in order that the resultant CNTfiber has excellent properties such as electrical conductivity, thermalconductivity, and mechanical characteristics.

The CNT dispersion liquid used in the presently disclosed method forproducing a CNT fiber can alternatively be prepared by dispersing theCNTs in the solvent using a different dispersion treatment method tothose described above.

[Carbon Nanotubes]

The CNTs used to prepare the CNT dispersion liquid can be the previouslydescribed plurality of CNTs that includes one or more collapsed CNTs.

[Dispersant]

No specific limitations are placed on the dispersant used to prepare theCNT dispersion liquid other than being a dispersant that can dispersethe CNTs and that is soluble in the solvent described further below. Thedispersant can be a surfactant, a synthetic polymer, or a naturalpolymer.

Examples of surfactants that can be used include sodiumdodecylsulfonate, sodium deoxycholate, sodium cholate, and sodiumdodecylbenzenesulfonate.

Examples of synthetic polymers that can be used include polyether diols,polyester diols, polycarbonate diols, polyvinyl alcohols, partiallysaponified polyvinyl alcohols, acetoacetyl group-modified polyvinylalcohols, acetal group-modified polyvinyl alcohols, butyralgroup-modified polyvinyl alcohols, silanol group-modified polyvinylalcohols, ethylene-vinyl alcohol copolymers, ethylene-vinylalcohol-vinyl acetate copolymer resins, dimethylaminoethyl acrylates,dimethylaminoethyl methacrylates, acrylic resins, epoxy resins, modifiedepoxy resins, phenoxy resins, modified phenoxy resins, phenoxyetherresins, phenoxyester resins, fluorine-containing resins, melamineresins, alkyd resins, phenolic resins, polyacrylamides, polyacrylates,polystyrene sulfonates, polyethylene glycols, and polyvinylpyrrolidones.

Furthermore, examples of natural polymers that can be used includepolysaccharides such as starch, pullulan, dextran, dextrin, guar gum,xanthan gum, amylose, amylopectin, alginic acid, gum arabic,carrageenan, chondroitin sulfate, hyaluronic acid, curdlan, chitin,chitosan, and cellulose, and salts and derivatives thereof. The term“derivatives” refers to conventional commonly known compounds such asesters and ethers.

Any one of these dispersants can be used or two or more of thesedispersants can be used as a mixture. From among such examples, thedispersant is preferably a surfactant, and is more preferably sodiumdeoxycholate or like, due to such dispersants exhibiting excellentdispersing ability toward CNTs.

[Solvent]

No specific limitations are placed on the solvent of the CNT dispersionliquid. The solvent may for example be water, an alcohol such asmethanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol,t-butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, oramyl alcohol, a ketone such as acetone, methyl ethyl ketone, orcyclohexanone, an ester such as ethyl acetate or butyl acetate, an ethersuch as diethyl ether, dioxane, or tetrahydrofuran, an amide-based polarorganic solvent such as

N,N-dimethylformamide or N-methylpyrrolidone, or an aromatic hydrocarbonsuch as toluene, xylene, chlorobenzene, ortho-dichlorobenzene, orpara-dichlorobenzene. Any one of such solvents may be used individuallyor two or more of such solvents may be used as a mixture.

[Dispersion Treatment]

In the dispersion liquid preparation step, a CNT dispersion liquid isprepared by subjecting a coarse dispersion liquid made by adding theabove described CNTs and dispersant into solvent to dispersion treatmentthat brings about a cavitation effect or dispersion treatment thatbrings about a crushing effect in order to disperse the CNTs. Thedispersion treatment that brings about a cavitation effect and thedispersion treatment that brings about a crushing effect are describedbelow in detail.

[[Dispersion Treatment that Brings about a Cavitation Effect]]

The dispersion treatment that brings about a cavitation effect is adispersion method that utilizes shock waves caused by the rupture ofvacuum bubbles formed in water when high energy is applied to theliquid. This dispersion method can be used to favorably disperse theCNTs.

Herein, specific examples of dispersion treatments that bring about acavitation effect include dispersion treatment using ultrasound,dispersion treatment using a jet mill, and dispersion treatment usinghigh-shear stirring. One of these dispersion treatments may be carriedout or a plurality of these dispersion treatments may be carried out incombination. More specifically, an ultrasonic homogenizer, a jet mill,or a high-shear stirring device can for example be suitably used.Conventional commonly known devices may be used as the aforementioneddevices.

In a situation in which the CNTs are dispersed using an ultrasonichomogenizer, the coarse dispersion liquid is irradiated with ultrasoundby the ultrasonic homogenizer. The irradiation time should be set asappropriate in consideration of the amount of CNTs and so forth, and isfor example preferably at least 3 minutes, and more preferably at least30 minutes, and is preferably no greater than 5 hours, and morepreferably no greater than 2 hours. Furthermore, the power is forexample preferably at least 20 W and no greater than 500 W, and is morepreferably at least 100 W and no greater than 500 W. The temperature isfor example preferably at least 15° C. and no higher than 50° C.

In a situation in which a jet mill is used, the number of treatmentrepetitions carried out should be set as appropriate in consideration ofthe amount of CNTs and so forth, and is for example preferably at least2 repetitions, and more preferably at least 5 repetitions, and ispreferably no greater than 100 repetitions, and more preferably nogreater than 50 repetitions. Furthermore, the pressure is for examplepreferably at least 20 MPa and no higher than 250 MPa, and thetemperature is for example at least 15° C. and no higher than 50° C.

In a situation in which high-shear stirring is used, the coarsedispersion liquid is subjected to stirring and shearing using ahigh-shear stirring device. The rotational speed is preferably as fastas possible. Furthermore, the operating time (i.e., the time that thedevice is rotating) is for example preferably at least 3 minutes and nogreater than 4 hours, the circumferential speed is for examplepreferably at least 5 m/s and no greater than 50 m/s, and thetemperature is for example preferably at least 15° C. and no higher than50° C.

It should be noted that the above-described dispersion treatment thatbrings about a cavitation effect is preferably carried out at atemperature of no higher than 50° C. The reason for this is in order tosuppress change in concentration that occurs due to vaporization of thesolvent.

[[Dispersion Treatment that Brings about a Crushing Effect]]

Dispersion treatment that brings about a crushing effect is even morebeneficial because in addition to of course enabling uniform dispersionof the CNTs in the solvent, dispersion treatment that brings about acrushing effect can also suppress damage to the CNTs due to shock waveswhen air bubbles burst compared to dispersion treatment that bringsabout a cavitation effect.

The dispersion treatment that brings about a crushing effect canuniformly disperse the CNTs in the solvent by causing crushing anddispersion of CNT agglomerates by imparting shear force on the coarsedispersion liquid and by further applying back pressure to the coarsedispersion liquid, while cooling the coarse dispersion liquid asnecessary in order to suppress air bubble formation.

When applying back pressure to the coarse dispersion liquid, althoughthe back pressure applied to the dispersion liquid may be lowered atonce to atmospheric pressure, the pressure is preferably lowered overmultiple steps.

Herein, in order to impart shear force on the coarse dispersion liquidand achieve further dispersion of the CNTs, a dispersing system may forexample be used that includes a disperser having a structure such asdescribed below.

Specifically, the disperser includes, in order toward an outflow sidefrom an inflow-side for the coarse dispersion liquid, a disperserorifice having an inner diameter d1, a dispersion space having an innerdiameter d2, and a termination section having an inner diameter d3(where d2>d3>d1).

In this disperser, when the in-flowing coarse dispersion liquid passesthrough the disperser orifice at high pressure (for example, 10 MPa to400 MPa, preferably 50 MPa to 250 MPa), the coarse dispersion liquid isreduced in pressure while becoming a high-flow rate fluid that thenflows into the dispersion space. Thereafter, the high-flow rate coarsedispersion liquid that has flowed into the dispersion space flows athigh speed inside the dispersion space while receiving shear force. As aresult, the flow rate of the coarse dispersion liquid decreases and theCNTs are favorably dispersed. A fluid at a lower pressure (backpressure) than the pressure of the in-flowing coarse dispersion liquidthen flows out from the terminal section as a CNT dispersion liquid.

Note that the back pressure can be applied on the coarse dispersionliquid by applying a load to flow of the coarse dispersion liquid. Forexample, a desired back pressure can be applied on the coarse dispersionliquid by providing a multi-step pressure reducer downstream of thedisperser.

As a result of the back pressure of the coarse dispersion liquid beingreduced over multiple steps by the multi-step pressure reducer, airbubble formation in the CNT dispersion liquid can be suppressed when theCNT dispersion liquid is finally exposed to atmospheric pressure.

The disperser may be provided with a heat exchanger or a cooling liquidsupply mechanism for cooling the coarse dispersion liquid. The reasonfor this is that by cooling the coarse dispersion liquid that is at ahigh temperature due to the application of shear force in the disperser,air bubble formation in the coarse dispersion liquid can be furthersuppressed.

Air bubble formation in the solvent containing the CNTs can also besuppressed by cooling the coarse dispersion liquid in advance, insteadof by providing a heat exchanger or the like.

As explained above, the dispersion treatment that brings about acrushing effect can suppress cavitation and can therefore restrictdamage to the CNTs caused by cavitation, and in particular damage to theCNTs caused by shock waves when air bubbles burst, which may be aconcern in some cases. Additionally, adhesion of air bubbles to the CNTsand energy loss due to air bubble formation can be suppressed, and theCNTs can be uniformly and efficiently dispersed.

One example of a dispersing system having a configuration such asdescribed above is a BERYU SYSTEM PRO (product name) produced by BeryuCorp. The dispersion treatment that brings about a crushing effect canbe implemented using a dispersing system such as described above bycontrolling dispersing conditions as appropriate.

[Viscosity of Carbon Nanotube Dispersion Liquid]

The viscosity of the CNT dispersion liquid is preferably at least 0.1Pa·s, and more preferably at least 0.3 Pa·s, and is preferably nogreater than 0.8 Pa·s, and more preferably no greater than 0.6 Pa·s. Thereason for this is that when the viscosity of the CNT dispersion liquidis at least 0.1 Pa·s and no greater than 0.8 Pa·s, the CNTs can befavorably spun in the spinning step described below, properties of theobtained CNT fiber such as electrical conductivity, thermalconductivity, and mechanical characteristics can be sufficientlyimproved, and the CNT fiber can be easily produced. The viscosity of theCNT dispersion liquid can for example be adjusted by altering theblending amount or type of the CNTs and the dispersant.

In the present disclosure, the viscosity of the CNT dispersion liquidcan be measured in accordance with JIS K7117-1 using a B-typeviscometer, with a temperature of 23° C., an M4 rotor, and a rotationalspeed of 60 rpm.

<Spinning Step>

In the spinning step, the CNT dispersion liquid is extruded into acoagulant liquid and spun. Specifically, in the spinning step, the CNTdispersion liquid is continuously extruded into a stirred coagulantliquid from a nozzle, a syringe, or the like, and a plurality of CNTsare spun in order to obtain a CNT fiber composed of a monofilament or amultifilament. The extrusion conditions for the CNT dispersion liquidcan be appropriately adjusted in accordance with the fiber diameter andso forth of a desired CNT fiber.

[Coagulant Liquid]

Herein, the coagulant liquid can be a solution in which the solvent andthe dispersant contained in the CNT dispersion liquid can be dissolvedor dispersed, and that enables assembly of the CNTs into a fibrousshape. Specifically, the coagulant liquid can for example be a solutionincluding any of N-methylpyrrolidone, N,N-dimethylacetamide, propylenecarbonate, formamide, N-methylformamide, water, methanol, ethanol, andpropanol. Note that the coagulant liquid is normally different from thesolvent in the CNT dispersion liquid.

Furthermore, the presently disclosed method for producing a CNT fibermay optionally include, after the spinning step described above, a stepin which the obtained CNT fiber is immersed and washed in water or thelike, a step in which the washed CNT fiber is dried, or a step in whichthe CNT fiber is stretched.

EXAMPLES

The following provides a specific explanation of the present disclosurebased on examples. However, the present disclosure is not limited tothese examples.

Example 1 CNT Synthesis

A coating liquid A for catalyst supporting layer formation was preparedby dissolving 1.9 g of aluminum tri-sec-butoxide, used as an aluminumcompound, in 100 mL of 2-propanol, used as an organic solvent, andfurther adding and dissolving 0.9 g of triisopropanolamine, used as astabilizer.

Additionally, a coating liquid B for catalyst layer formation wasprepared by dissolving 174 mg of iron acetate, used as an iron compound,in 100 mL of 2-propanol, used as an organic solvent, and further addingand dissolving 190 mg of triisopropanolamine, used as a stabilizer.

The coating liquid A described above was applied onto the surface of anFe—Cr alloy SUS430 base plate (produced by JFE Steel Corporation, 40mm×100 mm, thickness 0.3 mm, Cr 18%, arithmetic average roughness (Ra)approximately 0.59 μm), used as a substrate, by dip coating underambient conditions of a room temperature of 25° C. and a relativehumidity of 50%. Specifically, the substrate was immersed in the coatingliquid A and was held in the coating liquid A for 20 s before beingpulled up with a pulling-up speed of 10 mm/s. Thereafter, air drying wasperformed for 5 minutes, heating at a temperature of 300° C. in an airenvironment was performed for 30 minutes, and cooling was performed toroom temperature to form an alumina thin film (catalyst supportinglayer) of 40 nm in thickness on the substrate.

Next, the coating liquid B described above was applied onto the aluminathin film on the substrate by dip coating under ambient conditions of aroom temperature of 25° C. and a relative humidity of 50%. Specifically,the substrate having the alumina thin film thereon was immersed in thecoating liquid B and was held in the coating liquid B for 20 s beforebeing pulled up with a pulling-up speed of 3 mm/s. Thereafter, airdrying (drying temperature 45° C.) was performed for 5 minutes to forman iron thin film (catalyst layer) of 3 nm in thickness. Through theabove, a catalyst substrate 1 was obtained having the alumina thin filmand the iron thin film on the substrate in this order.

The prepared catalyst substrate 1 was loaded into a reaction furnace ofa CVD device maintained at a furnace internal temperature of 750° C. anda furnace internal pressure of 1.02×10⁵ Pa, and a mixed gas of 100 sccmof He and 800 sccm of H₂ was introduced into the reaction furnace for 10minutes (formation step). Next, the furnace internal temperature of 750°C. and the furnace internal pressure of 1.02×10⁵ Pa were maintainedwhile supplying a mixed gas of 850 sccm of He, 100 sccm of ethylene, and50 sccm of H₂O-containing He (relative humidity 23%) into the reactionfurnace for 8 minutes (growth step).

Thereafter, 1,000 sccm of He was supplied into the reaction furnace inorder to purge residual feedstock gas and catalyst activating material.Through the above, an aligned CNT aggregate 1 was obtained. The alignedCNT aggregate that was obtained had a yield of 1.8 mg/cm², a G/D ratioof 3.7, a density of 0.03 g/cm³, a BET specific surface area of 1,060m²/g, and a carbon purity of 99.9%. The aligned CNT aggregate 1 that hadbeen prepared was peeled from the catalyst substrate 1 to obtain CNTs 1.

<Confirmation of Presence of Collapsed CNTs>

The obtained CNTs 1 were subjected to fullerene insertion treatment bysealing the CNTs 1 in a quartz tube with isolated and purifiedfullerenes (C₆₀) and performing heat treatment at a temperature of 500°C. for 24 hours while maintaining a pressure of 1.07×10⁻³ Pa. As aresult of observing the CNTs 1 under a transmission electron microscope(TEM) after the fullerene insertion treatment, it was confirmed thatsingle-walled CNTs having collapsed structures were present asillustrated in FIGS. 1 and 2. Furthermore, when the number of collapsedCNTs was determined by TEM observation, it was confirmed that 32collapsed CNTs were present in 100 CNTs. The average width of collapsedsections in the collapsed CNTs was 6 nm.

<Carbon Nanotube Fiber Preparation>

The previously described CNTs 1 were added in an amount of 5.0 g to 500mL of 5 mass % sodium deoxycholate (DOC) aqueous solution, used as adispersant-containing solvent, to obtain a coarse dispersion liquidcontaining DOC as a dispersant. The coarse dispersion liquid containingthe CNTs 1 was loaded into a high-pressure homogenizer (product name:BERYU SYSTEM PRO, produced by Beryu Corp.) having a multi-step pressurecontrol device (multi-step pressure reducer) for applying back pressureduring dispersion, and the coarse dispersion liquid was subjected todispersion treatment at a pressure of 100 MPa. Specifically, the CNTs 1were dispersed by imparting shear force on the coarse dispersion liquidwhile applying back pressure and, as a result, a CNT dispersion liquid 1was obtained. Note that in the dispersion treatment, dispersion liquidflowing out from the high-pressure homogenizer was returned to thehigh-pressure homogenizer, and dispersion treatment was carried out inthis manner for 10 minutes. The prepared CNT dispersion liquid 1 had aviscosity of 0.58 Pa·s as measured by a viscometer (TVE-22H produced byToki Sangyo Co., Ltd.) at a temperature of 23° C. and a rotational speedof 60 rpm.

The obtained CNT dispersion liquid 1 was discharged from a nozzle havinga diameter of 120 mm and including 800 discharge outlets, each having aninner diameter of 150 μm, and was caused to coagulate in isopropylalcohol, used as a coagulant liquid, to obtain a fiber bundle(coagulated material). Next, the obtained fiber bundle was sufficientlywashed with water and dried, and was subsequently wound at a windingspeed of 20 m/s to produce a CNT fiber. A filament was removed from theprepared CNT fiber to obtain a CNT filament 1. The density of theobtained CNT filament 1 was measured to be 1.45 g/cm³. Next, a tensiletest (measurement conditions: temperature 20° C., relative humidity 65%,tensing rate 100%/minute) was carried out on the prepared CNT filament 1using a tensile test device (TENSILON). The tensile strength of the CNTfilament 1 was measured to be 940 MPa as an average value of fivemeasurements.

Example 2

In carbon nanotube fiber preparation, a CNT dispersion liquid 2 wasprepared by the same procedure as in Example 1 with the exception thatthe 500 mL of 5 mass % sodium deoxycholate (DOC) aqueous solution usedas the dispersant-containing solvent was replaced by 500 mL of 2.5 mass% sodium deoxycholate (DOC) aqueous solution, and the additive amount ofthe CNTs 1 was changed to 2.5 g. Furthermore, a CNT fiber was preparedby the same procedure as in Example 1 with the exception that the CNTdispersion liquid 2 was used. The viscosity of the prepared CNTdispersion liquid 2 was measured to be 0.15 Pa·s by the same method asin Example 1. Density and tensile strength of a CNT filament 2 removedfrom the prepared CNT fiber was measured in the same way as inExample 1. The CNT filament 2 had a density of 1.30 g/cm³ and a tensilestrength of 830 MPa.

Example 3

In CNT synthesis, a catalyst substrate 2 was prepared by the sameprocedure as in Example 1 with the exception that the pulling-up speedin application of the coating liquid B onto the substrate including thealumina thin film was changed from 3 mm/s to 6 mm/s. An aligned CNTaggregate 2 and CNTs 2 were prepared by the same procedure as in Example1 with the exception that the catalyst substrate 1 was replaced by thecatalyst substrate 2. The aligned CNT aggregate 2 that was obtained hada yield of 1.4 mg/cm², a G/D ratio of 2.1, a density of 0.03 g/cm³, aBET specific surface area of 680 m²/g, and a carbon purity of 99.9%. Thepresence of single-walled CNTs having collapsed structures among theobtained CNTs 2 was confirmed when the presence of collapsed CNTs waschecked in the same way as in Example 1. Furthermore, when the number ofcollapsed CNTs was determined, it was confirmed that 8 collapsed CNTswere present in 100 CNTs. The average width of collapsed sections in thecollapsed CNTs was 8 nm.

In carbon nanotube fiber preparation, a CNT dispersion liquid 3 and aCNT fiber were prepared by the same procedure as in Example 1 with theexception that the CNTs 1 were replaced by the CNTs 2. The viscosity ofthe prepared CNT dispersion liquid 3 was measured to be 0.44 Pa·s by thesame method as in Example 1. Furthermore, density and tensile strengthof a CNT filament 3 removed from the prepared CNT fiber were measured inthe same way as in Example 1. The CNT filament 3 had a density of 1.06g/cm³ and a tensile strength of 720 MPa.

Comparative Example 1

In carbon nanotube fiber preparation, a comparative example CNTdispersion liquid 1 was prepared by the same procedure as in Example 1with the exception that the CNTs 1 were replaced by multi-walled carbonnanotubes (MWCNTs; Lot. 1232 produced by Nanostructured & AmorphousMaterials Inc., BET specific surface area 57 m²/g). It should be notedthat collapsed CNTs were not present in the MWCNTs. The viscosity of theprepared comparative example CNT dispersion liquid 1 was measured to be0.042 Pa·s by the same method as in Example 1. Furthermore, whenpreparation of a comparative example CNT fiber was attempted by the sameprocedure as in Example 1 with the exception that the comparativeexample CNT dispersion liquid 1 was used, it was not possible to obtaina CNT fiber because the fiber broke.

Comparative Example 2 CNT Synthesis

A sputtering device was used to form a silicon dioxide film (carburizingprevention layer) of 100 nm in thickness on both front and rear surfacesof an Fe—Cr alloy SUS430 base plate (produced by JFE Steel Corporation,40 mm×100 mm, thickness 0.3 mm, Cr 18%, arithmetic average roughness(Ra) approximately 0.59 used as a substrate. Next, the sputtering devicewas used to form an aluminum oxide film of 10 nm in thickness and aniron film of 1.0 nm in thickness on only the front surface of thesubstrate on which the silicon dioxide film had been formed. Through theabove, a comparative example catalyst substrate was prepared. Next, acomparative example aligned CNT aggregate and comparative example CNTswere prepared under the same conditions as in Example 1. The comparativeexample aligned CNT aggregate that was obtained had a yield of 1.9mg/cm′, a G/D ratio of 6.5, a density of 0.03 g/cm³, a BET specificsurface area of 1,100 m²/g, and a carbon purity of 99.9%.

<Confirmation of Presence of Collapsed CNTs>

Single-walled CNTs having collapsed structures were not confirmed amongthe comparative example CNTs when the presence of collapsed CNTs waschecked in the same way as in Example 1.

<Carbon Nanotube Fiber Preparation>

The previously described comparative example CNTs were added in anamount of 2.5 g to 500 mL of 2.5 mass % sodium deoxycholate (DOC)aqueous solution, used as a dispersant-containing solvent, to obtain acoarse dispersion liquid containing DOC as a dispersant. A comparativeexample CNT dispersion liquid 2 was prepared by performing ultrasonicirradiation of the coarse dispersion liquid containing the comparativeexample CNTs for 1 hour, with a power of 300 W and a frequency of 20,000kHz, using a probe-type ultrasound device (product name: UX300, producedby Mitsui Electric Co., Ltd.). The viscosity of the prepared comparativeexample CNT dispersion liquid 2 was measured to be 0.09 Pa·s by the samemethod as in Example 1.

Furthermore, a CNT fiber was prepared in the same way as in Example 1with the exception that the comparative example CNT dispersion liquid 2was used. A filament was removed from the prepared CNT fiber to obtain acomparative example CNT filament. Density and tensile strength of thecomparative example CNT filament that was obtained were measured in thesame way as in Example 1. The comparative example CNT filament had adensity of 0.095 g/cm³ and a tensile strength of 240 MPa.

As clearly shown by the above, the CNT fibers in Examples 1-3, and inparticular the CNT fiber in Example 1, had high density and excellentmechanical characteristics.

INDUSTRIAL APPLICABILITY

According to the present disclosure, a carbon nanotube fiber can beprovided that has excellent properties such as electrical conductivity,thermal conductivity, and mechanical characteristics.

1. A carbon nanotube fiber comprising an assembly of a plurality ofcarbon nanotubes, wherein the plurality of carbon nanotubes includes oneor more carbon nanotubes having at least partially collapsed structures.2. The carbon nanotube fiber of claim 1, wherein the one or more carbonnanotubes having at least partially collapsed structures are present ina proportion of at least 5 in 100 carbon nanotubes.
 3. The carbonnanotube fiber of claim 1, wherein the one or more carbon nanotubeshaving at least partially collapsed structures have single-walledstructures.
 4. The carbon nanotube fiber of claim 1, wherein the one ormore carbon nanotubes having at least partially collapsed structureseach have a section into which fullerenes are not inserted upon beingsubjected to fullerene insertion treatment.
 5. The carbon nanotube fiberof claim 1, wherein an average width of collapsed sections in the one ormore carbon nanotubes having at least partially collapsed structures isat least 5 nm and no greater than 9 nm.
 6. The carbon nanotube fiber ofclaim 1, wherein the plurality of carbon nanotubes has a BET specificsurface area of at least 600 m²/g.
 7. The carbon nanotube fiber of claim1, wherein a density of the carbon nanotube fiber is at least 1.0 g/cm³and no greater than 1.5 g/cm³.
 8. A method for producing a carbonnanotube fiber, comprising spinning a carbon nanotube dispersion liquidcontaining a plurality of carbon nanotubes including one or more carbonnanotubes having at least partially collapsed structures, a dispersant,and a solvent by extruding the carbon nanotube dispersion liquid into acoagulant liquid.
 9. The method for producing a carbon nanotube fiber ofclaim 8, further comprising preparing the carbon nanotube dispersionliquid by subjecting a coarse dispersion liquid containing the pluralityof carbon nanotubes, the dispersant, and the solvent to dispersiontreatment that brings about a cavitation effect or a crushing effect inorder to disperse the plurality of carbon nanotubes.
 10. (canceled)